It is well known that the sensitivity of the human ear varies with frequency and the loudness level of the sound. This characteristic of the human ear was recognizedby Fletcher-Munsen and is represented by a family of sensitivity curves showing sensitivity as a function of frequency with the loudness level as the family parameter. Since the pitches of the notes on an electronic organ run from a frequency of 65.4 Hz at C.sub.2 to 2093 Hz at C.sub.7, waveshapes of constant amplitude at the lower frequencies will sound too soft to a listener compared to the same waveshape played in the higher octaves.
Pipe organs and electronical musical instruments which have individual and independent tone generators for each note have provides sounds of scaled intensity so that the listener perceives substantially constant loudness through the full keyboard range of the instrument. This presents a problem, however, in electronic organs which include a swell pedal for controlling the sound level of the entire instrument. Such a sound level control by operating equally on all notes tends to distort the otherwise carefully scaled loudness level compensation because the shape of the compensation curve is a sensitive function of the desired loudness level. An alternative loudness scaling technique which has been employed is to use a base-boost filter inserted between the tone generator and the sound system. Typically such a base-boost filter will amplify the fundamental frequency of the lowest note C.sub.2 by about 20 to 30 DB with the amplification factor tapering to unity gain for all notes above E.sub.3. However, the base-boost filter introduces unequal harmonic accentuation since the filter does not amplify the harmonics of the lower notes to the same extent that the fundamental is amplified. As a result, the tonal quality of the lower notes will be distinctly different from that of the upper notes as the result of the base-boost filter. The effect on the ear is an undesirable "boomy" effect for the low notes and particularly for the pedal tones.
Another method for obtaining loudness scaling in an electronic musical instrument is described in U.S. Pat. No. 3,908,504. The method therein described is particularly applicable to a computer organ such as described in U.S. Pat. No. 3,809,786 in which the amplitudes of consecutive points on a musical waveform are computed at equal time intervals in real time. The loudness compensation is accomplished by first determining the octave or half octave of a selected note actuated on the keyboard and then scaling the computed amplitude of each point by a scale factor determined by the relative sensitivity of the human ear to the fundamental frequency of notes within the octave or half octave. However, this quantization to either 12 or 6 notes produces steps in the apparent loudness which are easily heard and can be objectionable to the listener. Moreover there is no provision for changing the loudness compensation as a function of the loudness of the instrument as controlled, for example, by a swell pedal. An additional limitation is that no provision is made for loudness compensation changes as stops are added together in combination.
The present invention is directed to an improved loudness compensation control in a polyphonic tone synthesizer of the type described in U.S. Pat. No. 4,085,644. In the polyphonic tone synthesizer, the amplitudes of a fixed number of points defining one cycle of a musical waveshape are computed and stored in a register as a master data set. These points are then read out of the register at a rate determined by the fundamental pitch of the tone being generated to a digital-to-analog converter, which converts the sequence of points in the data set to an analog voltage which changes according to the desired waveshape of the tone being generated. The number of separate tone generators is limited, for example, to 12, which is normally the maximum number of notes that can be generated at one time in response to the ten fingers applied to the keyboard plus two foot pedals. These tone generators are reassigned each time a key is released and another key actuated on the keyboard. In order to control the attack, decay, sustain, and release characteristic of each generated tone, a time-shared ADSR generator is employed to modulate the gain factor of the digital-to-analog converter of each of the tone generators. Such an envelope generator is described in U.S. Pat. No. 4,079,650. The ADSR envelope generator is time-shared by all of the twelve tone generators of the polyphonic tone systhesizer. The ADSR envelope generator computes a digital value for each tone generator which changes in value in accordance with the desired changes in amplitude of the envelope of the tone being generated. The computation of the digital value involves an iterative computation starting with an initial amplitude value from which all subsequent values are computed. This initial value is a constant which determines the relative amplitude value computed by the iterative computational process of the ADSR generator.